DESCRIPTION: (Applicant's Abstract) Uridine has been shown to play a crucial role in modulating the cytotoxic effects of fluoropyrimidines in both normal and tumor tissues. The concentration of uridine in plasma and in tissues is tightly regulated by the activity of uridine phosphorylase (UPase) and by intracellular transport mechanisms. The applicant has utilized inhibition of uridine phosphorylase by benzylacyclouridine (BAU) to elevate uridine concentration in plasma and normal tissues to reduce the toxic effect of 5-fluorouracil (5-FU). During the past few years he has uncovered a series of differences between normal and tumor cells in the control and regulation of uridine levels that can be exploited to achieve more selective cytotoxicity in tumors, better protection of normal tissues that are primary targets of fluoropyrimidine toxicity, and thereby to improve the therapeutic index of 5-fluorouracil. The application focuses on the role of UPase in regulating the intracellular level of uridine in normal versus tumor tissue, on the characterization of this enzyme and its regulation, and on how the presence of a new phosphorolytic activity in human breast tumors can contribute to enhancing the selective rescue of normal tissues while maintaining the cytotoxic effect of fluoropyrimidines in human breast tumors. The proposed studies will also address some critical questions regarding the physiological function of uridine and uridine phosphorylase: 1) why the concentration of uridine in plasma, unlike other nucleosides, is so tightly regulated throughout different species, 2) why the induction of the neoplastic phenotype results directly or indirectly in the elevation of uridine phosphorylase activity, and 3) why breast tumor tissues have a BAU-insensitive, phosphorolytic activity that is not thymidine phosphorylase. To achieve these objectives the research will center on two main projects in progress: isolation and characterization of the protein(s) responsible for the BAU-insensitive phosphorolytic activity present in human breast tumors, and the evaluation of the role of UPase in uridine homeostasis and its effect on 5-FU antitumor therapy. Specific alterations of UPase genetic information will be achieved by: a) targeted disruption of UPase gene to nullify the UPase allele, and conversely, b) UPase gene transfer to elevate the UPase expression and activity in tissues.